1. Field of the Invention
This invention relates to a process for preparing predispersed chemicals in the presence of substantially clear serum. More particularly, the invention relates to a process for coprecipitating a rubber or plastic compounding chemical with a binder emulsion containing a latex of a rubber or plastic polymer to produce a homogeneous predispersed chemical composition and a serum having improved clarity.
2. Description of the Prior Art
In the manufacture of articles of rubber or plastic, it is necessary to add various rubber or plastic compounding chemicals, such as cross-linking agents, activators, accelerators, antidegradants, vulcanizing agents, stabilizers, and the like. Conventional procedures for incorporating these compounding ingredients have generally included the use of masticating equipment such as Banbury mixers or rubber mills. Using these procedures, either the ingredients are blended directly with the rubber or plastic, or masterbatches of these ingredients are added during mixing. These compounding ingredients are of necessity very fine powders so that they can be incorporated homogeneously with a minimum amount of mixing.
In the form of finely divided dry powders, these compounding ingredients present many problems to both the manufacturer and the user. Dry grinding produces dust clouds of reactive substances and increases the rate of evaporation of the more volatile ones. As dust or vapor, these finely divided powdered chemicals are a health hazard to all persons who must work in a polluted atmosphere from the dry grinding operation to the rubber compounding stage.
During all of the mechanical manipulations of these powdered chemicals, such as during packaging, transportation, unpacking, weighing and adding to the mixing vessel, the issuing dust clouds represent wasted chemical. This leads to inaccurate addition of chemicals which adversely influences the reproducibility of the cure rate and physical properties of the product from batch to batch. Since most of these compounding ingredients affect the resulting physical properties of the cured formulations to some extent, they are usually highly reactive, undergo hydrolysis and oxidation reactions, and some are undesirably deliquescent. These same reactions cause eye irritation and chemical burns on the skin and mucous membranes of workers exposed to these dusts and vapors. In addition, these reactions with water and air represent an unknown loss of activity which varies with each batch.
There are further serious problems with the use of dry powdered chemicals during the mixing stage. In addition to the chemical lost as a dust cloud or vapor during mixing, some may fall through the rolls during mill mixing. Some chemicals cause the rubber or plastic stock to become dry and boardy while others soften the stock to the point where it becomes sticky. When used in certain combinations some chemicals form low melting eutectics that cause poor processibility and prereaction. Other difficulties are encountered during mixing because these chemicals are generally polar substances that are not easily wetted by the high molecular weight nonpolar hydrocarbon polymers and agglomerate into large particles rather than fully disperse.
These problems associated with handling, mixing and adequately dispersing powdered chemicals into tough polymer matrices have plagued the rubber and plastics industries since their inception. Several processing techniques have been developed to alleviate these problems, but each has had its drawbacks. There have been three main approaches to producing dustless compositions for the rubber and plastics industries, viz. oiled powders or pastes, extruded and subsequently dried pastes containing latex binders, and masterbatches of selected chemicals at relatively low concentrations of chemical made by a coprecipitation process. Oiled powders or pastes, which are commercially available for most rubber chemicals, are sticky, messy to handle, break up the stock when they are added and are not true dispersions since they usually contain agglomerates. The extruded paste preparations, unless made with very low molecular weight or semi-fluid polymers, are very hard and difficult to disperse, particularly in short mixing cycles. These pastes usually contain soaps and surfactants to prevent destabilization during preparation which adversely affect such properties as water swell of the final product. They also contain the electrolytes and soaps used to prepare the latex. U.S. Pat. Nos. 2,640,088 and 3,012,985 disclose dried paste products of this type.
The latex masterbatching technique has generally been limited to the coprecipitation of a filler material with a latex, and optionally a plasticizer, for use in reinforcing or diluting the rubber or plastic stock. Examples of latex masterbatches of reinforcing agents or diluents, such as carbon blacks or silicas, can be found in U.S. Pat. Nos. 2,229,534; 2,964,490; 3,014,005; 3,317,458; 3,356,623; 3,640,940; 3,664,978; 3,700,620 and British Patent No. 781,152. In each of these patents, the filler material is blended with a latex and the resultng mixture is coagulated and this filler material is always present in amounts no greater than 400 parts per hundred parts of polymer in the latex.
Latex masterbatching has also been suggested for use in coprecipitating pigments and vulcanizing agents. Using this method, General Tire & Rubber Company has prepared predispersions of sulfur (Ko-Blend) and tetramethyl thiuram disulfide (Kure-Blend MT) in rubber in which the chemical was present in amounts no greater than 100 parts per hundred parts of rubber. In Canadian Pat. No. 457,146, pigments are coprecipitated with latex to form compositions comprising less than about 70 percent by weight of pigment. In U.S. Pat. No. 1,558,688, clay is coprecipitated with a rubber latex to prepare a composition having 90 percent by weight of clay.
None of these prior art latex masterbatching processes has been completely satisfactory. It has been found that when the filler material or pigment was coprecipitated with a latex, the serum which was formed upon coagulation was not clear. The cloudiness in the serum was due to particles of the filler not coprecipitated with or bound to any polymer and suspended polymer particles. This cloudy serum has generally been discarded resulting in the loss of large quantities of filler material and/or polymer. These losses can be reduced by the use of coagulation aids, such as animal glues, Swift Colloids, American Cyanamid Magnifloc 573C, duPont Amine 248, and cationic polyamine-type coagulation aids, such as Nalco 107. In U.S. Pat. No. 3,664,978, a process is disclosed for reducing the losses of product in the form of fine particles of elastomer-oil, elastomer-oil-carbon black, or elastomer-carbon black in the serum of a latex masterbatching process. The patented process comprises mixing additional coagulant and latex to the partially or completely coagulated mixture. U.S. Pat. No. 3,700,620 discloses a process for reducing silica pigment losses in the serum during the preparation of an elastomer-silica pigment masterbatch. The patented process is a complex process involving the use of wet silica pigment having residual or bound alkali content with an elastomer latex containing hydroxy groups wherein the filler material is present in the coagulum in an amount less than 100 parts per hundred of the elastomer. In both of these patented processes, the losses of filler or polymer in the serum were reduced by employing relatively large amounts of polymer in order to combine with and coat the filler particles. In Borg et al., Industrial and Engineering Chemistry, Vol. 38, No. 10, pp. 1013-16, it is found that in preparing a clay-latex masterbatch having less than 50 percent by weight of clay, retention of the clay by the rubber was increased, although complete retention was not achieved, by increasing the solids content of the clay-latex mixture prior to coagulation.
None of the suggested methods for reducing the losses of filler material or polymer in the serum which resulted from the prior art latex masterbatching processes is satisfactory since they require additional processing steps, and relatively large amounts of polymer are present in the coprecipitated product. It is desirable to obtain a product in which the concentration of compounding ingredient is as great as possible, first, since the polymer latex used to prepare the coprecitated product may not have the same composition as the rubber or plastic stock into which the ingredients ultimately are to be added and the amount of "impurity" introduced into the stock should be kept at a minimum, and second, since it is more economical to maintain the concentration of polymer as low as possible in order to lower the cost of the coprecipitated compounding ingredient for a given amount of compounding ingredient which must be added to the stock. U.S. Pat. No. 1,558,688 discloses a process using a high concentration of filler material, but that process was found to produce cloudy serum.